BACKGROUND OF THE INVENTION Technical Field of the Invention This invention relates to telecommunication systems and, more particularly, to a method of rapid handover of a mobile station between radio network controllers accessing an Enhanced General Packet Radio Service (EGPRS) network.

Description of Related Art The General Packet Radio Service (GPRS) is a packet-switched network for the transfer of high-speed and low-speed data and signaling between mobile stations (MSs) and other terminal equipment. The MSs may access the GPRS network through various Radio Access Networks (RANs). Althought GPRS was orignally designed to operate with the Global System for Mobile Communications (GSM), strict separation is maintained between the RAN and the core GPRS network, allowing the core GPRS network to be reused with other radio access technologies.

Enhanced GPRS (EGPRS) networks are enhanced with a technology called Enhanced data rates for GSM Evolution (EDGE). In such networks, MSs access the EGPRS network through an EGPRS Radio Access Network (ERAN), and the controlling entity within the EGPRS network is an Enhanced Serving GPRS Service Node (ESGSN). Various interfaces are now being defined between the ERAN and the

EGPRS network. The GSM 2.5 standard defined an interface based on the Gb interface and frame relay, while the Universal Mobile Telecommunications Systems (UMTS) standard (a third generation cellular standard relating to the evolution of GSM and EDGE) defined an IP-based interface utilizing the lu interface. EDGE has defined the Iu-PS'interface which is a packet-switched interface related to the UMTS Iu interface.

The existing and proposed interfaces between the ERAN and the EGPRS network, however, experience excessive handover delay when handing over a mobile station from one Radio Network Controller (RNC) to another within the ERAN. The resultant interruption time is a serious problem for real-time applications which are delay-sensitive. It would be advantageous to have a handover method, and an interface for use with the method that reduces the interruption time during inter-RNC handover to a level that is acceptable for real-time applications. The present invention presents such a method and interface.

SUMMARY OF THE INVENTION In one aspect, the present invention is a method of rapid handover of a mobile station between a Source Radio Network Controller (SRNC) and a Target Radio Network Controller (TRNC) in a Radio Access Network through which the mobile station accesses an Enhanced General Packet Radio Service (EGPRS) network. The mobile station is involved in an on-going call transferring Packet Data Units (PDUs) through the EGPRS network via the SRNC. The method includes the steps of notifying the TRNC by the SRNC that relocation of the mobile station to the TRNC is requested; reserving by the TRNC, radio resources required to serve the mobile station in the TRNC; and notifying the SRNC by the TRNC that relocation of the mobile station to the TRNC is approved. An lux interface may be established between the SRNC and the TRNC for direct communication of control signaling.

Alternatively, the control signaling between the SRNC and the TRNC may pass through the EGPRS network. This is followed by notifying the EGPRS network that a handover is taking place from the SRNC to the TRNC. The EGPRS then begins bi- casting PDUs to both the SRNC and the TRNC until notified by the TRNC that the

handover is complete. The mobile station is then handed over to the TRNC, and the EGPRS network is notified by the TRNC that the handover is complete.

In another aspect, the present invention is a Radio Access Network (RAN) for providing mobile stations with access to an EGPRS network. The RAN includes a first Radio Network Controller (RNC) that provides service to the mobile stations through at least one Base Transceiver Station (BTS). The RAN also includes a second RNC that provides service to the mobile stations through at least one BTS, and an lux interface between the first RNC and the second RNC for passing control signaling.

The lux interface may pass, for example, relocation messages between the first RNC and the second RNC when one of the mobile stations is handed over from the first RNC to the second RNC. In this manner, control signaling that is specific to the RAN does not have to be passed through the EGPRS network.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and its numerous objects and advantages will become more apparent to those skilled in the art by reference to the following drawings, in conjunction with the accompanying specification, in which: FIG. 1 is a simplified block diagram of an EGPRS Radio Access Network (ERAN) in which the lux interface of the present invention has been implemented; FIG. 2 is a signal flow diagram illustrating the flow of messages when performing an inter-RNC handover of a mobile station in a first embodiment of the method of the present invention when the lux interface is not implemented in the ERAN; and FIG. 3 is a signal flow diagram illustrating the flow of messages when performing an inter-RNC handover of a mobile station in a second embodiment of the method of the present invention when the lux interface is implemented in the ERAN.

DETAILED DESCRIPTION OF EMBODIMENTS The present invention is method of reducing the handover delay and the interruption time when handing over a mobile station from a Source RNC (SRNC) to a Target RNC (TRNC) in an EGPRS Radio Access Network (ERAN). As noted

above, it is critical for delay-sensitive, real-time applications that the interruption time be minimized.

FIG. 1 is a simplified block diagram of an ERAN 10 in which the lux interface 11 of the present invention has been implemented. The ERAN includes an SRNC 12 which controls base transceiver stations (BTSs) 13-15. BTS 14 is currently serving a mobile station (MS) 16 which is operating with terminal equipment (TE) 17.

Functionally, the SRNC includes a Radio Network Control Function (RNCF) 18 and a Radio Network Gateway (RNGW) 19. Control signaling passes through the RNCF while user (payload) information passes through the RNGW.

Also illustrated in FIG. 1 is a TRNC 21 to which the MS 16 is being handed over. The TRNC controls BTSs 22-24. Like the SRNC, the TRNC functionally includes an RNCF 25 and an RNGW 26. The SRNC 12 and the TRNC 21 are connected with the new lux interface 11. Both the SRNC and the TRNC are connected to the core EGPRS network (represented by the ESGSN 27) through an lu- PS'interface. Within each RNC, the Iu-PS'interface is divided into a control interface (Iu-PS'-C) which interfaces with the RNCF, and a user interface (Iu-PS'-U) which interfaces with the RNGW.

The lux interface 11 is normally utilized to transfer control signaling rather than payload information from the SRNC to the TRNC. In certain circumstances, however, payload information may also be transferred over the lux interface. For example, for loss-sensitive traffic, PDUs may be queued in the SRNC during the handover interruption, and then forwarded to the TRNC. Since it would be inefficient to relay those PDUs via the core network, the lux interface may be used to transfer queued PDUs to the TRNC.

FIG. 2 is a signal flow diagram illustrating the flow of messages when performing an inter-RNC handover of the MS 16 from SRNC 12 to TRNC 21 in a first embodiment of the method of the present invention when the lux interface is not implemented in the ERAN 10. There are 2 vertical lines below the SRNC and the TRNC, representing signaling in the control plane and the user plane. The control plane signaling is shown to pass through the RNCF 18 in the SRNC, and through the RNCF 25 in the TRNC. The user (payload) information passes through the RNGW

19 in the SRNC, and through the RNGW 26 in the TRNC. The ESGSN 27 represents the core EGPRS network.

At 31, a PDU (packet) transmission is in process. The MS 16 sends periodic measurement reports 32 to the SRNC 12 regarding the radio conditions. At some point, the signal strength gets weak enough to initiate a handover to another cell. If there is no lux interface, a Relocation Required message 33 is sent from the SRNC to the ESGSN and identifies the TRNC. The ESGSN sends a Relocation Request 34 to the identified TRNC. At 35, the TRNC performs a radio resource reservation procedure in order to speed up the handover process by identifying and reserving the necessary radio resources ahead of time. The TRNC determines the availability of radio resources required to serve the MS in the TRNC, and reserves the resources if they are available.

When the radio resources are reserved, the TRNC sends a Relocation Request Acknowledgment message 36 back to the ESGSN. The ESGSN then sends a Relocation Command message 37 to the SRNC. Both the Relocation Request Acknowledgment message and the Relocation Command are enhanced to carry information about the target cell resources that were reserved, the time slot on which transmission will be continued, and any other information needed by the MS for the radio access in the target cell.

If the TRNC determines that radio resources are not available, a failure indication is returned to the ESGSN in the Relocation Request Acknowledgment message 36. The ESGSN, in turn, sends the failure indication to the SRNC in the Relocation Command message 37. The SRNC then determines from the periodic measurement reports 32 which TRNC is the next best candidate for handover. The next best TRNC is then selected, and the process is repeated until the SRNC finds a TRNC capable of serving the MS, or it has to drop/drag the call.

The relocation signaling through this point does not have any impact on PDU transmission, which continues through all of these steps. Assuming 30 ms per message and 50 ms for the radio resource reservation procedure, a total of approximately 170 ms is spent preparing the network for handover.

After the Relocation Command message 37 is sent, the ESGSN starts bi-

casting the PDUs to both the SRNC and the TRNC at 38. This minimizes any interruption in the packet flow when the handover takes place. The SRNC then sends a Relocation Command message 39 to the MS, providing all of the control information needed by the MS to effectuate the handover (timing advance, time slot, uplink and downlink channels, triggering event, Temporary Flow Identifier, etc.). Approximately 20-40 ms is required to send the Relocation Command.

The MS then leaves its serving cell and enters the target cell. There is a possible interruption of 20-60 ms during the time that the actual switching is performed for the handover at 41 due to cell-switching and re-synchronization, if needed. In an alternative embodiment, the TRNC may buffer PDUs during the interruption to minimize packet loss. However, real-time applications are more delay- sensitive than they are loss-sensitive, and therefore there is no buffering in the preferred embodiment. When the first uplink PDU is sent from the MS to the TRNC at 42, it triggers a handover detection in the TRNC, and causes the TRNC to send a Relocation Detect message 43 to the ESGSN. This requires about 30 ms. This causes the ESGSN to stop the bi-casting of the PDUs at 44, and send them only to the TRNC.

The handover is then complete, and a Relocation Complete message 45 is sent from the TRNC to the ESGSN, requiring approximately 30 ms. The ESGSN then sends a Deallocate Resource message 46 to the SRNC which makes the radio resources previously used by the MS available to other users. This message does not contribute to the handover delay/interruption.

The entire procedure takes about 330 ms with only about 60 ms of interruption for the actual switching. Even with a safety factor of two, to account for possible channel errors and ERAN congestion, a handover interruption time of less than 120 ms should be achievable under all reasonable circumstances.

At some later time, the data session ends, and PDU transmission stops at 46.

The MS then switches from packet transfer mode to idle mode at 47, and begins acquisition of the Packet System Information (PSI) in the new cell at 48. The MS also realizes a possible Routing Area (RA) change and at 49, initiates an RA Update procedure, if required.

FIG. 3 is a signal flow diagram illustrating the flow of messages when

performing an inter-RNC handover of the MS 16 from SRNC 12 to TRNC 21 in a second embodiment of the method of the present invention when the lux interface 11 is implemented in the ERAN 10. Like FIG. 2, signaling through the SRNC and the TRNC is identified as control plane signaling or user plane (payload) signaling.

At 51, a PDU (packet) transmission is in process. The MS 16 sends periodic measurement reports 52 to the SRNC 12 regarding the radio conditions. At some point, the signal strength gets weak enough to initiate a handover to another cell.

Since the lux interface is implemented to carry control signals between the SRNC and the TRNC, a Relocation Required message 53 is sent from the SRNC directly to the TRNC using the lux interface. At 54, the TRNC performs the radio resource reservation procedure in order to speed up the handover process by identifying and reserving the necessary radio resources ahead of time.

When the radio resources are reserved, the TRNC sends a Start Bi-casting Request message 55 to the ESGSN, and the ESGSN sends a Start Bi-casting Acknowledgment message 56 back to the TRNC. The ESGSN then starts bi-casting the PDUs to both the SRNC and the TRNC at 57 in order to minimize any interruption in the packet flow when the handover takes place. The TRNC then sends a Relocation Confirm message 58 to the SRNC over the lux interface. The SRNC then sends a Relocation Command 59 to the MS, providing all of the control information needed by the MS to effectuate the handover.

Once again, there is a possible interruption of 20-60 ms during the time that the actual switching is performed for the handover at 61. When the first uplink PDU sent from the MS to the TRNC at 62, it triggers the TRNC to send a Relocation Detect message 63 to the ESGSN. This causes the ESGSN to stop the bi-casting of the PDUs at 64, and send them only to the TRNC. The handover is then complete, and a Relocation Complete message 65 is sent from the TRNC to the ESGSN. The TRNC then sends a Deallocate Resource message 66 over the lux interface to the SRNC which makes the radio resources previously used by the MS available to other users.

Like the first embodiment, the entire procedure takes about 330 ms with only about 60 ms of interruption for the actual switching. Even with a safety factor of two, to account for possible channel errors and ERAN congestion, a handover interruption

time of less than 120 ms should be achievable under all reasonable circumstances.

Thus, the advantage that the lux interface offers is not in further reducing the handover delay, but rather in reducing the signaling load on the core EGPRS network, and in further separating radio access functionality from the core network.

If the first handover attempt fails, for example because the TRNC does not succeed in its attempt to reserve radio resources, the Relocation Confirm message includes an indication of failure. The SRNC then repeats the same procedure with other TRNCs until it finds a TRNC capable of serving the MS, or it has to drop/drag the call. The lux interface is very efficient in this scenario since a significant number of messages do not have to go through the core EGPRS network.

At some later time, the data session ends, and PDU transmission stops at 67.

The MS then switches from packet transfer mode to idle mode at 68, and begins acquisition of the Packet System Information (PSI) in the new cell at 69. The MS also realizes a possible Routing Area (RA) change and at 70, initiates an RA Update procedure, if required.

It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description. While the ERAN and method shown and described has been characterized as being preferred, it will be readily apparent that various changes and modifications could be made therein without departing from the scope of the invention as defined in the following claims.